Federal authorities say licensed medical marijuana users can’t own a gun. A September letter from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives advises gun dealers that knowingly selling to a medical marijuana user is against the law, even in one of the 17 states with a medical marijuana law on the books.

Anyone who legally buys a gun fills out ATF Form 4473, which asks whether the person is “an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance.”

If a medical marijuana user answers no, he or she has broken the law, according to the feds.

If the answer is no and the firearms dealer knows the person uses medical marijuana — or has “reasonable cause to believe” so — but sells the gun anyway, then both parties have committed a crime.

Medical marijuana advocates call the measure a violation of rights.

“It really just reiterates the federal government’s policy of discrimination,” said Jeremy Rupinski, director of the Genesee County Compassion Club.

“We don’t have this policy for prescriptions or for other homeopathic medicines.”

Others think the stance is common sense.

“I wouldn’t want to sell them a gun and be responsible for them shooting somebody,” said Randy Bullock, owner of Bullock’s Guns & More in Genesee Township.

Bullock said he doesn’t mind the times he’s had to turn customers away because they smell of marijuana or admit to using the drug medically.

“I’m not losing my license over a thing like that.”

But gun dealers aren’t required to ask specifically about medical marijuana and, unless misused, it’s not something that would come up on the criminal background check.

Bullock suspects some medical marijuana users lie and are able to buy guns.

But unless they smell like pot or a medical marijuana card falls out of the customer’s wallet, what’s a dealer to do?

“I’m not going to sit here and interrogate them,” he said.

Both medical marijuana and firearms have widespread popularity in Genesee County. As of August, the state had approved 6,800 medical marijuana applications from residents here. And there were 15,000 people registered to carry a concealed weapon, which doesn’t even account for all gun owners.

The issue of medical marijuana and guns has come up at the Genesee County gun board, a collection of local law and government officials charged with deciding who can carry a concealed weapon.

This year, the board decided — by a 2-1 vote — to allow a 36-year-old medical marijuana user to keep his concealed weapon privileges. Those voting to allow the license did so based on assurance from the U.S. attorney’s office that it had no intention of prosecuting CCW holders violating federal law.

The Sept. 21 letter from the ATF was “really just a clarification” of the agency’s stance on the issue, said ATF spokesman Drew Wade.

The department had received several inquiries from licensed firearms dealers claiming customers were using medical marijuana cards as forms of identification.

Though medical marijuana is illegal under federal law, users operate under the assumption that U.S. Department of Justice officials will make good on their pledge not to prosecute users.

But gun dealers may not be as willing to dance around the law.

“Dealers are heavily regulated by the ATF, and that’s their livelihood. So gun dealers try to be extremely cautious to be sure they’re on the right side of the line,” said Steve Dulan, a board member of the Michigan Coalition for Responsible Gun Owners and a professor of firearms law at Cooley Law School.

Though the coalition doesn’t have a position on the issue of medical marijuana users owning guns — and abstains from doling out legal advice to gun owners — Dulan said the issue could end up getting hammered out in the courts, and people could argue their Second Amendment rights are being violated.

“It’s not likely that you’re going to get one of these folks essentially volunteer to be a test case,” Dulan said.

Rick Thompson, editor of Michigan Medical Marijuana Magazine, said the directive is an attempt to slow the spread of medical pot laws to other states. At least seven new states could see the issue on a ballot in 2012, he added.

“It’s a scare tactic,” he said.

Thompson and Rupinski said they don’t expect feds to start investigating or prosecuting users just because they own guns.

“This is not a proactive policy. They are not going to come to your home to see if you have guns,” Thompson said.